The 7.5cm (75mm) Sonnar for the Tenax II

7.5cm f4 Sonnar for the 35mm 24X24 Zeiss Tenax II camera

[To see my earlier entries on the Tenax II camera below, go to: Tenax II action camera ].

Zeiss Ikon offered a selection of only four lenses for its Tenax II camera. The 7.5cm f4 Sonnar was its sole telephoto lens.

Tenax II with 7.5cm Sonnar and 433/17 albada finder for 4cm & 7.5cm lenses

Despite its limited range of lenses and the restricted development of a system of photography, everything Zeiss made for the Tenax II was first-class and of the same quality as accessories sold for the Contax. This is particularly true of its optics. The Tenax II and its lenses were not cheap ( the 7.5cm Sonnar was $120.00 in 1939), nor were they intended for the common amateur. Rather, they were aimed at prosperous, middle-class families who wanted a fast-action camera with interchangeable lenses for recording family and recreational activities.
The Carl Zeiss Jena 7.5cm Sonnar is a four element optic similar to the 135mm f4 Sonnar for the Contax. It is the same design as the 7.5cm for the Movikon movie cameras. Zeiss also made a number of these lenses in Robot mount. Construction is satin chrome over brass. The focusing is a double helicoid so the lens does not rotate during focusing. This is true for all the Tenax II lenses. The need to couple to the gears that drive the wedge focusing optics for the rangefinder may have required such a mounting. The non-clickstop diaphragm has settings to f22, and is double, resulting in a beautiful circular opening at all settings and an even spacing of the stops. The focusing lever sets below the lens and rotates counter clockwise for about 30 degrees to carry the focus from infinity to 1.5 meters (63 inches or a little over five feet).

Since the Tenax II shoots pictures that are 24 X 24 instead of the almost universal 24 X 36, the 7.5cm Sonnar offers framing that would be the equivalent of a 94mm lens in a full-format 35mm camera.

The Thiele database of Zeiss lens production indicates that a total of 1025 7.5cm Sonnars were manufactured in four groups. This is a much higher number than the rare 2.7cm Orthometar, the Tenax II’s only wide-angle, but as many as 50 of these shipped out in Robot mounts and it is possible that most if not all the last lot of 275 manufactured in 1940 went to the Kriegsmarine (German war navy) and ended up off at sea. So the 7.5cm Sonnar may be rarer than production numbers would suggest. However, a few 7.5cms have shown up with numbers that do not fit into Thiele's production series, so who knows?.
At least one 7.5cm has been seen without its rangefinder arm and equipped with a ring-driven helicoid. The author has seen two late 7.5s with Kriegsmarine inventory markings on their lens rear mounts. In both cases the numbers have a “MF” prefix. "M" stands for "marine," but I have not seen a consensus on what the "F' stands for. Some naval lenses have inventory numbers that just start with a "M."

The mount is the three-tang Tenax II standard and the back barrel seats well into the camera body. To mount, set the shutter speed to 1/50th or slower, line up the red dot of the lens with the red-marked pin, set the barrel in, rotate counter-clockwise about 30 degrees and the lens clicks into place. To remove, depress the top lever and rotate clockwise. The projecting arm of the lens houses the two wedge-shaped lenses that connect via gears to the outer helicoid. These lenses line up in front of the camera’s rangefinder window. As you focus, these lenses rotate, one clockwise, the other counter-clockwise, to cause the RF image to aline.

As is true of all Tenax II lenses. the 7.5cm Sonnar takes 35.5mm screw-in filters and 37mm slip-on accessories. Zeiss offered two Proxars, 1 and 2, in the 37mm slip-on type. The Proxar 1 mounted on a 7.5cm, allows focusing from 41 to 24 inches. To determine accurate focusing, one would need to set the camera on a tripod and use a tape measure.

Zeiss Ikon also offered a Contameter set (#1339) for the Tenax II but whether its lenses or focuser would work with the 7.5cm Sonnar is unknown. The focuser and the #50 lens from the #1341 set for a Super Ikonta B will work nicely at about 21 inches. See entry on closeup work with a Tenax II: Closeups with the Tenax II

The 7.5cm f4 Sonnar illustrated here came with the dent to the front rim you see, eliminating any possible use of screw-in filters. Otherwise, this blemish has no effect on the lens’ performance. However, when I received the lens, it had another problem: the helicoid would not turn. This was not expected. After completely disassembling the helicoid and the rangefinder arm, I was able to determine the problem lay in the gears that connect the wedge lenses to the outer helicoid.

The focusing gears for the 7.5cm Sonnar disassembled. The two small
gears rotate the front RF lens counter-clockwise and the larger gear
rotates the rear RF lens clockwise.

They set into the arm casting and were binding against the casting. The gears rode on pins that follow an eccentric path when rotated. By turning the pin screws slightly, I was able to shift the gears so they no longer rubbed the casting. But reassembly was a challenge. The gears and the two wedge lenses had to be exactly right to the tooth in order for the rangefinder image to appear to move horizontally as you focus the lens. Even one gear tooth off for either lens would result in a RF image that would appear to move up or down on a diagonal. Once that was corrected, I had to rethread the inner helicoid onto the outer so that the lens would set at infinity and the screw holes lined up. That also took time. In the end, it took me two days to determine the problem and fix it. Fortunately, none of this affected the optics which are in good condition with only one small scratch on the front element and no cleaning markings.

Claire & her mother.

Bloodroot in bloom.

Penns Creek near Poe-Paddy. Note the flare.

Easter egg hunt.

Repairman's mark from distant past.

Assembly number on focus tab.

Kriegsmarine inventory number on rear mount partly removed by previous owner.

Assembly number on helicoid. Note the careful engraving.

Performance is about what one can expect for a uncoated lens. It flares in direct sunlight but yields nice, sharp images indoors or pointed away from the sun. It is so small and compact that weight is never a problem.

 Framing is another issue. Zeiss Ikon offered two options: one, a combination 40/75 albada finder, the other a mask that clipped onto the studs on either side of the Tenax’s finder window. Good luck finding either possibility!

Older lens on left, later lens on right.

For the student of the manufacturing changes that occur over time and due to war, here are two 7.5cm Sonnars, both in Tenax II mount for comparison.

Both lenses appear to be the same, but a closer examination reveals a range of differences.

The first (2246175) is an early production lens, one of the first 200 built in three lots in January 1938—only a few months before the public release of the Tenax camera. It is the earliest serial number in my records. It is an export lens intended for sale to either the British or American market.

Features:
1) The engraving of the the beauty ring is bolder and larger than the later lens, and not as sharp. It does not include slots for a spanner wrench.
The distance scale is in feet and begins with “Inf.”
“Made in Germany” is engraved on the outer edge of the lens mount.
The chrome finish appears to be a high quality. The ding on the front-rim has not resulted in any chrome flaking. A poorly-done straightening has resulted in a crack in the rim. One can screw in a filter part-way.

The second lens (2543494) is from Thiele’s third lot of 150 dated to April 1939. With the Second World War less than five months away, most of this lot may have been intended for the German military.

The engraved letters and numbers on the beauty ring are finer, smaller and better defined. The beauty ring includes slots for a spanner wrench.
The distance scale is in meters and begins with “∞”.
No indication as to the country of manufacture other than the “Carl Zeiss Jena” marking.
The chrome finish is poorer with a bit of flaking where the filter rim was dinged and and a rub-though mark on the barrel. On the other hand, the lens-release shows no pitting, while the earlier lens does.
It is engraved “MF818” on the rear of the lens mount. This has been ground down to hide the number, but the letters and number are still easy to see. This is a military mark indicating a lens for the Kriegsmarine (“war navy”). In the picture you can see how the mark was applied with a hand-held engraving tool.

The later lens with the distance scale in meters

The early lens with distance scale in feet.

Made in Germany engraved on rim of early lens.

Closeup of military engraving on lens flange

The glass on both lenses are in fair condition. The early lens has clear front glass with the slightest hint of cleaning marks, but the back element has a haze that hints at the possibility of fungus. The later lens has a scratch on the front surface but no apparent cleaning marks. The front glass is not as clear as the earlier lens but the back element is beautifully clear and clean. Take your pick as to which will be the better user.